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MOCVD Routes toTl2Ba2Can-1CunO4+2nSuperconductor and Dielectric

Insulator Thin FilmsB. Hinds, D. Studebaker, J. Chen, R. Mcneely, B. Han, J. Schindler, T.

Hogan, C. Kannewurf, T. Marks

To cite this version:B. Hinds, D. Studebaker, J. Chen, R. Mcneely, B. Han, et al.. MOCVD Routes to Tl2Ba2Can-1CunO4+2nSuperconductor and Dielectric Insulator Thin Films. Journal de Physique IV Colloque,1995, 05 (C5), pp.C5-391-C5-406. <10.1051/jphyscol:1995546>. <jpa-00253908>

JOURNAL DE PHYSIQUE IV Colloque C5, supplkrnent au Journal de Physique 11, Volume 5 , juin 1995

MOCVD Routes to T ~ ~ B ~ ~ C Z I ~ - ~ C U ~ O ~ + ~ ~ Superconductor and Dielectric Insulator Thin Films

B.J. Hinds, D.B. Studebaker, J. Chen, R.J. McNeely, B. Han, J.L. Schindler, T.P. Hogan, C.R. Kannewurf and T.J. Marks

Northwestern University, Dept. of Chemistry, 2145 Sheridan Rd., Evanston IL. 60208-3113, U.S.A.

Abstract. The evolution of HTS device technologies will benefit fsorn the development 01' MOCVD (Metal-organic Chemical Vapor Deposition) routes to high quality HTS I'ilms as well as to those of insulators with low dielectric loss and close HTS lattice matches. Reviewed here are research efforts at precursor design focusing on Ba sources. A novel low pressure TGA technique is used to compare volatilities of MOCVD precursors and to quantif'y the role of' gas phase diffusion in film growth. TO form high quality T12Ba2Can-1C~1104+2n (n = 2.3) films, BaCaCuO(F) films are first deposited by MOCVD using the liquid precursors Ba(hl'ai?*rncp. Ca(hfa)2*tet, and solid Cu(dpm)2 (hfa = hexafluoroacetylacetonatc, clp~n = dipivaloyl~netha~iatc. mep = methylethyIpentaglyme, tet = tetraglyme). The film growth process is shown to he Inash transport-limited, and an interesting ligand exchange process is identified. The supei.conductu~; TBCCO phase is formed following an ex-situ anneal in the presence of T120 ar tentperau~i-cs from 820-900°C. Transport properties of TBCCO-2223 films include a Tc as high as I15K. J c of 2x105 A/cm2 (77K), and Rs of 0.35mQ (5K, 10 GHz). The MOCVD growth of low loss, lattice-matched dielectric NdGa03, PrGa03, SrzAlTaOg, and SrPrGa04 f'il~ns is also discussed. High quality YBa2Cu307-, films have been grown upon MOCVD-dm-ived PsGaO? suhsri.a~cs.

1. INTRODUCTION

The discovery of high temperature superconducting (HTS) oxides promises the scali~ation of many novel

thin film device concepts. Among these are Josephson junction-based devices for highly sensitive

magnetic field detection or ultra-fast computation.[l] Due to the inherentIy low surface resistance.

passive microwave devices such as filters and delay lines are other important applications of HTS

materials.[2] Many of these HTS-based devices also require insulating, la~rice-matched. low-loss

dielectric oxide films. To fabricate such devices on a large scale, an efficient inethod to PI-oduce high-

quality thin films of oxide superconductors and dielectrics must he developed. MOCVD has man).

important attractions ill terms of ability LO coat large areas with high throughput ~ ~ n d e s highly oxidi~iny

conditions. However, thc MOCVD technique also offers many challenges. Thcse include the synthesi.;

and characterization of suitably volatile precursors, understanding precursor decomposition and clystal

growth mechanisms, and efficient reactor design. Of particular intercst here is ~ h c gsou~th of' t h c

T I B a C : ~ ~ u ~ l.~imily ol. 5~1pc1-cond~~ciors \\;hich have aillong thc h i ~ l ~ c s l k n ~ l \ \ i ~ c . l . i l i c . : i l lcinpcl.at~ii.c-\

l 1 ? 5 K ) . l ? ] -hi. gso\\th 01' these material. b y MOCVD pscscnts ~ h c chailcngc o r I ' ln i l ing a i ~ ~ i l - l h l \

i ' ( r l ~ ~ i l ~ harium source ai-rii ~ ~ n d c r s t a n d i ~ ~ r r l x co~tiplcs phasc I-clatii~nships rrt thc 'TI UaCaCuO I TBCCO.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1995546

C5-392 JOURNAL DE PHYSIQUE IV

family of superconductors. Presented here are the efforts to: 1) Develop suitably volatile alkaline and

rare earth precursors. 2) Characterize their volatility and vapor pressure temporal stability with a novcl

reduced pressure thermogravimetric analysis. 3) Characterize the MOCVD growth process for

BaCaCuO(F) thin films. 4) Grow high quality Tl2Ba2Ca,-lCunO, thin films in a post annealing process

and characterize transport and microwave properties. 5) Grow high quality lattice-matchcd diclcctrics hy

MOCVD for use with HTS materials.

2. EXPERIMENTAL

2.1 Metal-organic precursor synthesis and TGA characterization

Ba(hfa)z*mep, Ca(hfa)ytet, and Cu(dpm)2 (hfa = hexafluoroacetylacetonate, dpln = dipivaloylmetha~iate,

mep = methylethylpentaglyme, tet = tetraglyme) are prepared as described in the literature.[4]

B a ( d ~ m ) ~ is synthesized anhydrously from the reaction of Ba metal with Hdpm in T H F solution. To

measure the relative volatilities, sublimation rates of precursors were measured in a T A Inst~x~ments STD

simultaneous thermogravimetric-differential thermal analysis (TGA-DTA) apparatus. The TGA-DTA

directs a horizontal carrier gas t-low across 3.4 mm deep alumina sample pans mounted on microbal;ince

arms. Pressure is adjusted by N2 tlow rate (50-100 sccm) and by a throttle valve in the connection to thc

14 CFM direct drive pump.

2.2 MOCVD growth

The research MOCVD reactor used in this study (Figure 1) is of a straightforward horizontal design. and

details are described elsewhere.[5,6] Metal-organic precursor reservoirs are individually heatcd i n

thermostated oil baths, and carrier flows are mixed in a common manifold. T o aid in uniformity of

deposition rate, a quartz laminar tlow chamber is utilized. The 3:l aspect ratio of width-to-height in this

tlow chamber reduces thermal buoyancy effects, thus enhancing the stability of can-ier tlow.[7] A SiC-

coated graphite susceptor is positioned at the end of the flow chamber and is angled at 8.7" to aid

uniformity of deposition along the length of the substrate. The susceptor is heated by a 6 kW water-

Ar carrier gas Oxygen I I

@ Mass flow controllers @ Needle valve @ 3-way bypass valve

fl ~ ~ ~ Z % Z ~ e r v o i r s

Water bubbler

b T o vacu urn

Liqu~d N2Traps IR lamp

Figu~.c 1 . Schematic of research MOCVD reac~or-

cooled IR lamp, and temperature is monitored by a K-type thermocouple on the surl';icc of the s~~sccp to r .

prcssurc is controlled by throttle valve. To find stoichiometry and deposition rates. the ~netal content 0 1 '

films is determined using an AtomScan 25 ICP atomic emission spectrometer. In the bypl-oduct t~.apping

s[udy. a 0'' C trap is placed on the bypass line 10 cm downstream from the hypass ~ . a lvc . A small ulhc

ful.nace, at 400" C , i s placed around the bypass line hetwcen the trap and hypass vulvc. Oxidizcr is

inuoduced at the common precursor manifold.

2.3 TlzBa2Ca,-lCu,04+2n phase formation

Due to the high volatility and extreme reactivity of TI20, it is difficult to grow films in an i , l - .~ i ru process

(i.e., one not requiring a post-anneal). MOCVD derived BaCaCuO(F) films arc thus ~cac ted with an

equilihriurn pressure of T120 from a bulk TBCCO superconductor pellet of maintained at telnperaturcs

between 820-900°C. The pellet is prepared from presintered T1203, BaO, CaO. CuO powders with cation

ratios of 2212 or 2223. The film is supported face down over the pellet with gold foil and sealed in a

crimped gold foil crucible to minimize TI20 loss. The atmosphere of the furnace is purged with purified

O2 or 10%,02/Ar before the crucible is sealed. The temperature is ramped slowly at 2"Cfmin to 700'C to

drive off any absorbed water, then quickly ramped (2O0C/min) to reaction temperature. Hold times vary

from 0.3 LO 15 h.

2.4 Thin film characterization

X-ray difurac~ion data were collected with a Rigaku DMAX-A diffractometer using Ni-filtered Cu K, radiation. Film thickness was measured by a Tencor P10 profilometer. Fluorine content was moniiored

by Hitachi 4500 SEM using Cambridge Scientific EDX analysis with light element window. Magnetic

susceptibility measurements were made using a Quantum Design SQUID. T, and J,. on 8Op1n widc

patterned bridges, were measured using the 4-point probe instrumentation descrihed elsewhere.[X]

3. DISCUSSION

3.1 Precursor design

Any efficient MOCVD process relies critically on the availability of high purity metal-organic precursors

with high, stable vapor pressures. An important approach to realizing such precursors is to niinimizc

molecular oligirnerizalion by saturating the metal coordination sphere with sterically hi11del.cd and/or

fluorinated ligands. Of equal importance is that the precursor has suitahle reactivity at deposition

temperatures without prior gas phase decomposition. Until the discove~.y 01' HTS matcsials. ~ h c '

development of' alkalinc and rare earth metal MOCVD precursors had been largely neglected.

Generally, the four classes of' volatile metal compounds are organometallic. halide. alkoxide, and

coordinarion co~npounds (such as b-diketonatcs). Volatile organometallic compc>unds 01' alkaline and

rare earth metals are known, however they tend to be cx[remely oxygen-sensitive. scndcring tsanspo1.t in

an oxidative cnviron~ncnt dif'ficult. Halide CVD has been utilized in HTS matc~.i;lls. howcvcl. dil'l'ic~~llic\

exist in transporting the relatively nonvolatile halides and in adjusting thc the~~modyna~n ics to I'o~.nl

osides.[9] Alkoxidcs are utilized in the growth of oxide thin filnis (such as BaTiO? \vi t l i

i C 1 1 I l . 0 Ho\\,cvs~.. Inan>, alkosides tcnd to hc i~ l igo~ncr i c (cil>ciiall! \ i . ~ ~ l i noii-hl~lL!-

lisanils). Iiavc l o ~ v \.apor pscssu~-cs, and arc air scn>itivc'. P-dik~'ti)n;ltes 1'01-111 c o ~ i l p l c ' \ ~ ~ 1~~1t11 1110,t 1lir1;11\

JOURNAL DE PHYSIQUE IV

and offer great versati l i ty in chemical

modification. One stratcgy for modil'ication is

to use the sterically encumhered r-hutyl g~,oup

in t h e d i p i v a l o y l ~ n c t h a n a t c ( d p ~ n .

1,1,6,6tetra1nethylt1eptnl7e2.4dio11atc or tmhd)

3 ligand. However, with electropositive ~ncral, having large ionic radii, the dpm ligan~i is no1

B. effective in reducing latticc oligo~nc~-ization.

I \ - T ~ U S , dpm complexes ot' Ba. ss and LI have *o 0 o Oh

low volatilities. T o achieve suitahlc vapol.

pressures these dpm co~npounds must he heatcd

to nea r the r e s p c c ~ i v c decomposi t ion Figure 2A. Ba(hfa)a-mep, a stable liquid barium Precursor used in temperatures, in unstahlc vapor this study for the growth of TBCCO thin films. B. Novel ketoiminate ligand forming a readily sublimable Ba complex. pressure characteristics and unseliahle C V D

processes. In particular, a stahle prccurso~. for

Ba would he highly desirable.

T o reduce lattice cohes ive energies, large bulky fluorinated l igands can he used.

Bis(tetradecailuorononanedionato)aquobarium(II), B a ( t d f r ~ d ) ~ * H ~ O , had been employed for the grow111 01'

BaF2 films at a source temperatures 40°C less than typically used f'or Ba(dpm)?.[ l 1 1 Another very

attractive strategy is to use the electron withdrawing ability of fluorinated ligands to promote thc

coordination of neutral, electron-rich, ancillary ligands, thus further saturating the metal coordinatio~i

sphere. Meinerna, er al. employed a coordinated polyether around the ~ a ? + ion in Ba(hfa)?*tct.l 121 Thcy

found that the compound sublimes as a monomer at temperatures 100" C less than Ba(clp~n)?. Thih

technique has been similarly applied in the Ba(tdfnd)? system with the coordination of tctsaglymc.[ 131

Of particular importance here is the low melting point of this compound. A liquid prccursos climinalc

vapor pressure instabilities that can arise I'rom varying surface area of powders due to sintcring et'i'ccts.

Another approach to lowering the precursor melting point is to modify the coordinating pc)lycthc~.. Using

unsymmetrio polyethers, Neumayer, e t al. developed volatile Ba(hfa)~*methylbutylhcxagly~nc with a

melting point as low as 52"C.[14] The related Ba(hfa)a)z*mep (mep = methyletliylpentaglyme) (Figure 2A)

has a melting point of 110°C and is used as a liquid Ba sourcc in the prescnt study 01' TBCCO growth.

A disadvantage of the fluorinated precursors is that at deposition te~npcraturcs 01' 3.50-780" C.

fluorides are incorporated in the growing film from decomposing ligands. However, without thc

electron-withdrawing fluorinated ligands, polyethers do not form stahle complexes. An altcrnativc approach is to attach polyethers directly to the b-diketonate ligand to stahilizc the coordination of rhc

polyether. Work in this laboratory used polyethers attached to ketoirninate ligands and demonstratctl

useful volatility in this class of complexes for the growth of BaPhO? f i l ~ n s hy MOCVD. A rolarcd compound with the polyether spanning two P-ketoirninate ligands has also bccn rcccntly synrhcsi~sd

(Figure 2B). The crystal structure shows the compound to be coordinately sat11ratcd.l 161

3.2 Reduced pressure thermogravimetric analysis

In ordel. to have a controlled MOCVD process, the vapor pressures ot' the ~nctal-ol.ganic pl-cClll..\Ol..\ mtlst

he 111ol.oughly characterized. However , many HTS metal-organic precursors have relatively low vapoi.

p~.essu~.cs (< 0.3 T o r s under usual deposition conditions) which makes vapor prcssurc measurement

dil't'icult. T o s tudy vapor pressures in the lo 10-3 Torr range, it is pos.sihlc to uac high vaclturn

cvaporation rates combined with the Knudsen-Langmuir equation. For vapor pressures grc2rc.1. that 5

Ton.. i t i s possible to use either transpiration inethods or to measure the hoiling poinr dil.cctly. TO

measure vapor pressures hctween and 5 Torr , the pressure must he directly ~ n c a s ~ ~ r c d with ;I

manometer using a rigorously sealed and evacuated chamber.[l7] This method can encounter great

experimental dit'iiculties d u e to the formation of decomposition products and other volatilc ilnpilritics.

With great care , reliable data c a n be obtained.[ l8] However , data describing di1~i'usi~)n of' M O C V D

precursors cannot be acquired using this method. Under typical M O C V D conditions, total system

pressures range from 1 to 2 0 Tor r and non-equilibrium diffusion effects can be significant. A useful

technique is to measure the evaporat ion rate of a precursor using thel~moglavimetl.ic analy.\i> ( T G A )

techniques under reduced pressure. From this dynamic procedure. information concerning ;)pcl.ational

vapor pressure and diffusion can be obtained.

T h c evaporat ion rate in high vacuum relates to equilibrium vapor pressure hy the Knudscn-

Langmuir eq uation,[l9] j = a PE (M/2xkT)l/2 ( 1

where 1 is the evaporation rate per unit area, a is the condensation coefficient, PE is thc eqililihri~lm

partial pressure, M is the molecular weight, k is the Boltzman constant, and T is temperature. At higher

pressures o f inelt gas, diffusion effects become important. The Langmuir equation hecomes.

where P, i s the pressure of the compound at distance x from the surl'ace, and D is rhc dit'l'usion

coefficient. At the limit ot' high total pressures (small D), this becomes Fick 's first law ot' difiusion.

There a re several examples of reduced pressure T G A being used to study the volatility 0 1 '

MOCVD precursors. Takahashi , et al. used the classic Langmuir relation to descrihc the volariliry 0 1 '

organic monomers for use in vapor deposition polymerization.[20 They note a suhlimation sarc dccrcasc

with increasing total pressure, which they attribute to a pressure dependent a term i n e q . ( I ) . AL ~ h c

lowest attained total pressure of Torr , they assumed a = I to ca lc~ l la te the equilibrium vapor

pressures. However , the assulnption 01' a = 1 is not generally valid, and or can vary hy scvc~.al orde1.s 01'

magnitude for organic powders .[ l7] Stlictly speaking, a is independent o i inest ?as p~.cssurc ~ n t ! cannot

be measured 1.01. unclcl'ined powder surfaces without knowledge oC equilihl-ium \.apor ~ ~ C S S L I I . ~ . The

observed decrease in evaporation rate with total pressure is more likely dit 'f~lsion-dcpclide~~t. ah descsihcd

hy ec1.(2). Chou. r t ol., measurcd the dynamic evaporation rates ol 'Ba(dpm)?. Ca(dpin),. Cu(dpm)- , at 20

Ton. total p r ~ s s u r c , which is near typical low pressure MOCVD condition.s.l2 I I They scporrcd u decrea.sc

in suhlimation rate with time which they attributed to sintering and changc in hciglll 01' the prcc111.~(1i.h 111

the sample pans. Due to alnbiguity in sample preparation in relation to samplc height and area. clil'l'usic~ll

~ 1 2 1 ; ~ n c r c i1,>1 O I > ~ : I I I I ~ ~ I . L L I I ~ I the cl'l.c'.ts o [ ' s ~ n t e ~ - i n g wcl-c I I C I ~ ~ i i . ~ c c ~ ~ ~ ~ c c l . 111 11115 L . O I ~ I I . I ~ I L I I ~ L I I I \ \ L , I . L . ~ ~ I I , I J

~ilil[)lc ~.cclu~.c.il pl.c.~,urc TGA cspcrimi.nt rhar ailo\\;s mca , sL~rc~ncn~ 01' ~ 1 1 ~ - ~ I I . L I L I ~ L . I 0 1 ' tllc ~ l i l l u \ ~ o ~ l

0 5 10 15 20 75 30 35 -1U O D U S 10 1 5 - 0 5 , i r 1 4 L O

Pressure (Torr) Diffus~on d~stance x f r r i r n ]

F~gure 3 A. Effect of N2 pressureon suhlimuun rare ut Cu(dpm)>ir 1 lj'Ci5 T b r r N ~ pre\\use r l i nrcns~ircd by rrduaetli pressure TGA. Under typical MOCVDco~ldltluns, dlffUaon r&c& dormnate preculsor t~ansport- B Subllnvasitrrr l a ~ e 01 Cu(dp)2 a t 120aC/5 Torr N2 pressure as n fun~r lun or drffuslon dlsrance x D~fkszon durance, x, 1% ~alculazad from "owel&t tlrncs pan helght. Inset shows Ftck's law dttiuslon nludcl ta fmd POD, w ~ r i ~ n~t ldr r iy~ng soltd Iint :L tn rcl

expenmental data

coeffic~cnt t imes the. equllihnum vapor pressure of MOCVD precursors. Thia c l i lc~ws for a c o n t ~ ~ o l ~ d

iomparison of precursor volatll~ty in the MOCVD cnv~runrncnt. W ~ t h the equ~librium 1 apol pressuie t r l

the coirlpound known, the cilflus~on c v e f f l ~ ~ z n i can be calculated,

The rate of sub l imdt lon in n TGA rxpzrtment is caIcu1ated.h-om the clc~lvati\~c o t xve~ght jvith

respecr ru L.ne divlded by molecular weight R I I ~ the drea o b the cruclbla. Figurc 3A i t laws that tor

I cpresc.ntative Cu(dpm)l, (fie iatc of evapor &on 15 s~gn i i ~cant ly reduced ,is t h e .inrb~ent preisurc 1s

increased, which lnd~cates chat diifuslor-1 1s d lln~ltlrig factor ~n the transport ot thesz precursors LVlth

d~ffusion effects b e ~ n g ~rnpor-tdnt u n d ~ these cond~tlons, the distance that the prccursol n m r t tlavol i i u m

the surface of the powder to the top ui the cructhlz I S 'I 5igplhcdnt t'tctrlr Figurc 3B shows tha

evaporation rate as a funt Ljon of distailoe from the s o l ~ d plelulstlr ~ U I ~ L ~ C I : t o [he c C I ~ L . o t the i ~ u i l b l ~ : i x =

we~gbt/hll-welght x cructblc height) Ovzr a distance ol 3.4 mrn, a 3-told dcc-rcasc: In sublimahon-late-19

seen at 5 Torr total pressure The same reduction In evaporatloIt rate 1s been tor 11qu1d precursors t ~ i

slinilar volatility, thus sintel-~ng over time is not the dominant causaot vo la t~ l~ ty reductton To n~adet 1hl.e

subl imat io~ rate, n o t e that the measured weight loss rate is the flux 01 precursor frorn the top o t tho.~)aii

into the carrier stream a,). We assume that t h ~ s rate 1s proportional to the pressure o i precursor at t h ~ :

plant: on top of the pas (P,) a n d to the dliiusion coeff~clcllt (Dl (ec1..(3)),

1~ = KDPx, P, =jY / KD (31

where K IS a constant dependent on the geometry of the cnlcrble curibcc with I c5prcl LO the ~ L I I I I ~ J streart11

At slow ternparature ramp I - ~ L C S , stcady-state conditior~s mist, and j, is cqua l rc) thr t l t u of' l>!.i'cul.sris I~.i.>rn~

the surface of the powder t o l h e Lop u f the pan. This Flux i s tiescribed by FicL's i'irsr Iitu, t d i t i i t u , r u ~ ~ ~

( eq.(4)). Substitu tlng P, from (3) illto (4) yields (51,

220~C 180'C 140°C 1 OO'C 60 C 3 , : : : I

- - (I)

N-

a, -C e

- 1 C 0 .- +

E -? .- - I) 3 -3 (I) V

F~gurr -I. Cornpariso~i of volatilities of six HTS MOCVD precursors dcrtrmincd by rcduccd prc\\urc TG.A C ~ I I I C ~ I I I O I I \ : i Tun total preahure: N2 flowing at 100 sccIn: ternperaturc ranp rate 01' 1 .S'C/rni~i. S u l ~ l i l ~ l ; ~ ~ i o l ~ r i i ~ t h i u c I I O I . I I I ; I I I / ~ C J 10 C I

d ~ l f u s ~ o ~ i dihta~ice ot 1.7 rnm. Inflection points show tneltitig poi~irs of Uir var~ous precur\orr\

where PE is the equilibrium vapor pressure of the precursor a1 the surface of' llic po\vde~. and s is rlic

distance from the powder surface to the top of the pan. A two parameter curvc 1'11 ~)f'cspcr.irnenral j, r.\. s

yields the product DPE and K. Unfortunately, this single experiment cannot separate vapor pressurc ant1

diffusion effects. Since the equilibrium vapor pressure of Ba(hfa)2-tet is known from dit.cct rnanomctric

rneasurements,[lX] the resulting diffusion coefficient calculated from PED is 1 . 5 ~ 1 0 ~ 1n7/s a1 115" C. 5

Torr N2. Thus, it can be seen that a boundary layer of several millimeters l'rom precursor surhcc L O

carries stream can have dramatic effects on the evaporation rate. From thesc reduced prcssurc TGA

experiments, it is readily apparent that diffusion plays a dominant role in the transport of thc metal-

organic precursors. This is of particular importance when designing transport cvapo~.ators 1'(:1. all

1IOCVD reactor. These effects can also be seen in the slopes of Arrhenius plots 01' the In(suhlimation

rate) vs. 1IT for Ba(hfa)z*tet which yield activation energies for the suhlimation process. A1 total N7

Pressures of 6.0 Torr, the activation energy for Ba(hfa)?*tet vaporization is 1.70 kJImc11: a1 -1.0 TOIT. 1.50

u/mol; and at 0.1 Torr 1.23 kJ/mol. These data can be compared to the enthalpy ol' vapori/atrori 01. I .OS

kllmol from the equilibrium vapor pressure data.[l8] As the diffusion coef'l'icicnl is gcncrally dcscrihcd

lo be thermally activated, the activation energy for the sublimation process would he expected to he thc

of the vaporization enthalpy and diffusion activation energy terms. Since the suhlima~ion rate i.\

S ' r o n ~ l ~ dependent on diffusion, equilibrium vapor pressure data alone cannot hc used to I'ind thc oplimal

"mPerature for a given transport rate. Typically, for precursor transposl, the conditions of 1 . 1 0 ~ ~.alc.

pressure, and temperature must be empirically determined for a given cvaporalor geometry.

(25-398 JOURNAL DE PHYSIQUE IV

1.2 1 Having charactelized thc difl'usion cfl'ect\

in the TGA sample pan. the evaporation rate vs.

temperature dala can be normalized to a dil'fusioll

distance of 1.7 mm. The relative volatilities (11'

some common HTS MOCVD precursors arc

shown in Figure 4. Thc f'l~~c>rina~ed Ba(hfa)l*t~t

and Ba(hl'a)2*mep sources asc mosc volatile [ha11

Ba(dprn)z and suhlime at temperatures far below

the respective decomposition temperaLures. This

o 20 40 60 80 100 characteristic is critical t'or MOCVD proccss Time (h)

control s ince decomposing precursor will Figure 5. Vapor pressure temporal stability of imponant Ba produce an vapol- pressure, and l.ilrn MOCVD precursors at film growth temperatures. Sublimation rate is measured at 5 Torr N7. Between points, the TGA is stoichiometry will be i~ncontrollahle. Also ot' - hackfilled with 1 atm N2, where the sublimation rate is negligible. The co~nlnonly used Ba(dpln)2 temporal stability is

importance is that Ba(hl'a)z*mep is a liquid a1

poor compared to the fluorinated precursors. deposition temperatures (which aids in growtll

reproducibility) and is 3 times more volatile than

Ba(hfa)2*tet at 120°C.

T o demons t ra t e the s tahi l i ty ol

Ba(hfa)z*mep and Ba(hia)2*tel compared ro

Ba(dpm)2, the evapc~ra~ion rates wcre measured

at various times over the course ol' I'OLII. days ai

film growth temperatures. The TGA chamher

was backfilled to atmospheric N2 pressurc

between measurements to minimize weight lash

1 0 (s ince the diffusion 01' thc precursor a l

20 60 loo I2O 14' atmospheric pressure is negligible). As is readily Argon carrier flow over Ba(hfa) .tet (sccrn) apparent in Figure 5. the Iluorinated alkalinc

Figure 6. Growth rate of BaO(F) films as a function of carrier gas flow rate through the Ba(hfa)~*mep precursor reservoir. earth precursors have ln~ lch improved vapol' Conditions: 5 Torr total pressure; 350 sccm 02 /H20 oxidizer; pressure temporal whereas ~ ~ ( d ~ ~ ) ~ total flow 500 sccm with Ar dilution; 500' C substrate temperature. Cross hatches show the relative precursor weight decomposes substantially over the course of' 21) Ioss at 25 and 150 sccm carrier flow. Inset shows gas phase stoichiometry during film growth as measured by precursor hours' weight loss. BaCaCuO film stoichiomeuy is measured by ICP In summary, reduced pressurc TGA is a atomic emission spectroscopy.

powerful tool to mcasure the volatilities of

various precursors under MOCVD film growth conditions. At typical MOCVD psessuses, dil'fusio~i

effects are substantial and reactor design must reflect this. Fluorinated harium polyether adducl

precursors are demonstrably more volatile and stable for MOCVD film growth.

3.3 Growth of BaCaCuO(F) thin films

Thc MOCVD process is complex and can be described i n ~ h c followitlg stcps: 1 ) vi~l;~tili~..ation (11' rhc

prccus.sor i n ~ o [he carrier stream, 2) transporl of the precursor to thc h o ~ scaction zonc. 3 ) surl'ac?

absasp~ion 01' [he precursor and other reac~ive species, surl'ace migra~ion, and ciccompositio1-1. 4 ) s11rt';lcc

and fo~mation of [he crystalline solid film, and 5 ) I-emoval of the organic by-pl.oducls. In Inally

MOCVD processes, the second step is known to be rate-limiting, and reacrclr design col~sideratiollh

provide a constant supply of gaseous precursor over the area of the deposition zone. As described in the

Exper.imenta1 Section, the present MOCVD reactor design is employed to provide a ilnil.orm supply 01'

precursor over a large substrate area. To determine whether the film growth rate ih limiicd by ~ I L I S \

transport effects, it is necessary to elucidate the relationship of precursor partial pressure 10 depc)sillon

rate. Figure 6 shows that as the carrier flow through the precursor reservoir is ~ncseased from 25 to 150

seem (with constant total carrier flow) the deposition rate increases approximately linearly. U ~ i d c ~ .

conditions of constant total carrier flow rate, a doubling of the precursor llow rare should ideally douhlc

the concentration of precursor in the carrier stream and hence double the deposition rate. H[-)wcve~. an

increase in deposition rate by only a factor 01' 1.5

is seen when the precursor stream flow sate ia

doubled. With a diffusion limited suhlimatioi~

rate, the precursor carrier gas will not 1.eacI1

saturation and higher flow rates will dilute the

pressure of precursor. Hence thc deposition ratc

should not be directly proportional to precursor.

flow rate. At flows much higher than 150 sccm

(the maximum for the present apparatus).

deviation from the ohserved linear ~.clationship

would be expected. Also oI' importance are thc

crosshatched points in Figure 6 thai show thc

relative precursor weight loss 1'01. precursors 31 thc

reservoir flow rates of 25 and 150 scan . Both thc

deposition rate and precursor rate increase by a

factor of 3.3. The deposition rate is directly

proportional to precursor transport rate and hcncc

to the precursor partial pressure in the total car.rier

stream, as expected for a mass t~.an.sport-lin~iteci

process.

As suggested hy the TGA experimcn~s. thc

diffusion of the precursor into the cai.rier srrualn is

a limiting factor in the suhlimatio~l rate and i i

would thus be difficult to achieve equilihl.ium

precursor pressures in the carrier stream. From thc ~ I J b ' I I I I I I I I ' I I I I ' I precursor weight loss and currier gas vi~ltr~ue, thc

200 iOO 400 500 600 700 'O0 ideal gas law partial pressure ol' Ca(hl'a)2=tet i \ Substrate Temperature ( 'C)

~i~~~~ 7, MOCVD dewsirion rate subsuate estimated to he -0.005 Torr in thc carrier stseam. BaCacuo thin films using B a ( h f a ) z * m e ~ , Ca(hfa)z'tet, while equilibrium v ~ p o l . prcss,~r.c i s near 0 3 C ~ ( d p 1 1 1 ) ~ precursors. Co~id~tioris: 5 T ~ r r total pressure: 350 scclll ositiizc.r no\v: 150 seem tr,lal c:,r,-icr flow, .t,) wiu1 T ' x ~ . [ l 81 Thus, ihe spcciiics L I I ' ~.c.scr-\-r)il. deign 111

H:O a i d C), oxidizing h u c l u ~ l . B) Orily 0: oxidizcr C) Oril! Lerlns or diff.usion il i51a l , i . c Lil.c. a l l ,ml ,o , - t ; in l 02 o x l d ~ ~ e r arid C u ( d p ~ n ) ~ precursor.

C5-400 JOURNAL DE PHYSIQUE IV

factors for HTS MOCVD processes. If film growth is mass transport-limited, then thc gas p h a s ~

stoichiometry should be the same as that of the deposited film. The inset in Figurc 6 shows that thc g3$

phase stoichiometry as measured by precursor weight loss is identical Lo that 01' the deposited I'ilm 3 ,

measured by ICP.

To further demonstrate the mass-transport limited character of BaCaCuO growth. dcposition ralc

versus substrate temperature data are shown in Figure 7. The precursors used werc B a ( l ~ l ' a ) ~ * r n c ~ ,

Ca(hfa)2-tet, and C u ( d ~ m ) ~ , and the oxidizer stream was O2 and H20 . It can he seen that a transport-

limited growth regime begins at temperatures near 350°C. This is interesting hecause 1'01- the MOCVD 01'

YBa2Cu307 using non-fluorinated Y(dpm)3, Ba(dpm12, Cu(dpm)2 precursoss. a mass transpol.~-li~nircli

regime was found at temperatures greater than 60O0C.[22] The decomposition ol' thc hl'a ligands I (

apparently more facile, which is surprising in view of the [act that fluorinated ligancls produce mosc

Lewis acidic metal centers which in turn form more stable adducts with the polycther glymes. Since thc

deposition rates here are similar to those observed in other studies (-20 nm/min)[221, thc depression ill

the onset temperature for mass transport-limited kinetics is not due to a lower mass tsansporr ratc. 111

principle the effect of H 2 0 on the decomposition reaction could bc grcat sincc i t could act 2s a proto11

source to form volatile Hhfa byproducts. To explore this possibility, thc ahovc exper i~ncnt way

conducted without water in the oxidizing stream. The same general rcsults wcrc ohscrved with mas\

transport-limited deposition beginning near 400°C (Figure 7B). The dcposition charucrcristics o l

C ~ ( d p m ) ~ under the conditions of Figure 7B are markedly different. At 400°C. therc is essentially n o Cu

deposition. A possible explanation is that the decomposition products of B a ( h f ' ~ ) ~ * m e p and Ca(hfa)2*tc~

form Cu(hfa)2 which is then etched from the film. There is precedent for Cu(hfa)-, heing l 'o~med in thc

selective etching of PdICu surfaces.[23] The temperature vs. deposition rate plot for C u ( d p ~ n ) ~ in Figi11.c

7C shows a rather constant growth profile that gradually increases with temperature. This is cc)nsistcnt

with the results of Schmaderer, et al. 122lwhich reveal a very shallow increase i n CuO deposition I- at^

from 400°C-900°C. The diminution of CuO deposition at 400" C seen in Figurc 7B requires the prescncc

of fluorinated precursors and the absence of H2O. Note that in the experiment shown in Figure 7C.

metallic Cu deposits at lower temperatures from Cu(dpm):! alone.

To test the hypothesis that copper can be etched from BaCaCuO(F) films as Cu(hf'a)?, thc volatilc

metal byproducts of Ba(hfa)2-mep, Ca(hfa)z0tet, Cu(dpm)2 codeposition were trapped at 0°C sho~.tly ai~tr.

passing the gaseous mixture through a resistively heated tube at 400°C. Thc trapped solids wcrc then

analyzed by mass spectrometry. When the products of the MOCVD reaction at 400°C without H 2 0 i n t h t

oxidizer stream (as in Figure 7B) were analyzed, both Cu(hfa):! and Cu(ht'a)(dpm) wcrc psescnr 1111dts

conditions where CuO etching was prevalent. However, when the products 01' he MOCVD rcaction ~ l t

400°C with H 2 0 in the oxidizcr strcanl

Table 1. Typical MOCVD growth conditions for BaCaCuO films (as in Figure 7 A ) wcrc analyzed. having 2: 1:2 cation stoichiometry

Cu(hfa):, was also ohserved (withon[ . - Precursor bath temperatures 115", 86", 112" C for Ba,Ca,Cu C u O etching). Thus, ligand cnchangc Precursor carrier flow 50- 100 sccm each 021H20 flow 1001250 sccm is operative without simultaneo~ls Total pressure 5 Torr etching oI'CuO. 11 is itiipoi-(ant to ~ioti' S L I ~ S L S ~ L C tcmperalurc 500°C Growth rate 1.2 ~ r n l h s that n o Cu(h1 ' ;1)~ i,\ c c n when th~. Substrate ( 11(j) single c~ys ta l LaAlO? cai.~.ier strc;1111 I.\ c o ~ i ~ l c ~ i s c ~ l W ~ L I I O L I ~

passing through the hcarcd rcacrion /one.

From thc study ol' dcpohirion raw

as a t'unclion 0 1 ' pl.ccurso~. rran.\port and

substrate t e ~ n p c r a t ~ ~ r e . 11 is I'ound that the

genera l mechanism of' B a C a C u O ( F )

MOCVD growth is Inass transpol-r-limited

with thc present precussor sysrcln. H?O in the oxidizer stream has little cffcct o n

film growth rate. Ligand cxchangc

between p~.ccu~.so~.s is ohscr\~cci and t1ic1.c

is evidence I'or a C u O ctcliing process.

Although the present M O C V D

process is chemically complex. thin films

of uniform composition over large a~.cax

can be readily grown. The conditions I'C)I.

the growth ot' BaCaCuO(F) I'ilms with

2: 1 :2 Ba:Ca:Cu cation stoichiometry a1.c

shown in Tahle 1 . T h i s s i ~ n p l c seactol-

design affords a uniform deposition ratc

F~~~~~ 8. SEM In,crograp~l of an MOC-D.~~~~~~~ ~1.2212 fill,, in a 1 6 cm2 area). Crys~a l l inc phascs 01'

showing chiil.i~~leristi~ platelel morpl~ology. CaO, BazCuO, and BaF? arc ohservcd in

the depos i tcd ~ h i n I'illns by x-ray

dil'f'raction. D u e to the use of fluorine-containing metal-organic precul.sors. f luo~. ides arc readil)

incorporated into the film. T o reduce the fluoride content, H 2 0 is used in he oxidizer stream to Iluoridch

as volatile H F . T o remove all fluorides in-situ, high deposition temperatures (780°C) arc rcquircd.

However, as described in the TI-anneal section (see below), the fluoride conrent in the BCCO precursor

film is not a critical factor, and deposition c a n be carried out a t lower t e ~ n p e r a t ~ ~ . c s to enhance I'ilm

morphology and ro grow larger area films.

3.4 Formation and characterization of T12Ba2Can-1Cun04+2n thin fiIms

The equilibrium pressure of TI20 over bulk T1-2223 at phase formation temperatures is ncal 10 Tosr.1231

This relatively high pressure combined with the extreme reactivity of T120 reiidcrs an in-situ MOCVD

process a daunting challenge. Therefore to form the T B C C O superconducting phascs, the M O C V D -

derived BaCaCuO(F) (BCCO) precursor films are annealed over hulk T B C C O pellets, rherchy providing

the equilibrium T l 2 O vapor pressure as in eq.(6). The subscripts of the reactant Ba~Ca,I-lC~[lOx(l.il1,~,

refer to thc carion stoichiomctry (n = 2,3), ol' the BCCO film.

B a ~ C a I 1 ~ l C u n O x ( l i l , n ~ + T l ~ 0 ( ~ ) - > T ~ ~ B ~ ~ C ~ , , . I C L I , , O ~ , ~ , , ( I , ~ , , , I ( 6 )

*~1~o;~,, ,r120p) + 02 1 - 1

C5-402 JOURNAL DE PHYSIQUE IV

This T120 pressure over the bulk pellet is strongly depcndcnt on the ~lnncallng L C ' I ~ ~ ~ ~ ~ L L I I . L ' a11J 0,

pl-cssurc as is seen by the ecluilibrium in eq.[7). ~v i th higher T120 pl.cssurcs acliic\.;~hlc ar Itrwcs (I7

pi-c.ssurcs. Thc. Iclnperarilrc required ro t'ol-n~ [he TI-22 1 - 7 phase di.c.1-cascs inarkc.JI> .ih 0- pr.cxs~1r.2 1,

decreased. with Tl-2117 phase fortning at 870k 5" C and 895 + 5' C lor i05;i and 1005 01. respccr~vcly.

The TI-pool. Tl- 1212 phasc is commonly szen oursidc of this range of ci)nciiticrns. That rhc T I 2 0 p~.c.\\uj.~

control is key to TBCCO phase stability is strongly supported by the rcsults 0 1 ' :\sclagc. r j r 01.. En L L V L I -

Lone furnace experiments ~vhere TI20 pressure is conrrollcd.[?-l] It is imp~Ir1ant to n o ~ c thar n o tlurrridc~

x c derected in anncaied MOCVD films hy windowlcss EDS, Auger arizlysis. ril. XRD. BCCO prcctr~-strl-

films grown at 50()"C contain negligible fluoride after the T1-anneal. Thc tl~~oriclc is prc\irmahly ~-erno\,e~!

during rhc anncal as TIF ~vhich is \tolatile and thermodynamically favored.[7-51

T o form the TI-7223 phase. the BCCO precursor film is annra lcd ovcl- ~1 I:?:?:?

Tl7O3:BaO:Ca0:Cu pellet in 1 0 8 O1 at 820°C for 13 h. Lokver 0 2 pressures hzlp to stabilize the gri~n.rh

of the T1-2223 phase[26] and long annealing tirtles (12h) are required. Films art. shiny ro the eye. and th?

aurface morphology is dominated by evidence of a melt during phase fn~mat ion 2s s h o w ~ l in Figurc. Y .

Large platelets of 20pm dimensions and 500,4 step heights. as measured by pr-ot'ilometry, arc seen. .A 0-

2 8 x-ray diffraction scan (Figure 9 A ) shows an essentially phase-pure T1-2223 sample.[27] Only th?

(001) peaks are observed, indicating that the film is highly c-axis oriented. Thc 8-sucking crrrvc ( 3 t'lll-thcl.

measure of c-axis orientation) exhibits a full width at half rnaxirnurn of' 0.8" tf igi~rc. 9B) which I\

comparable to values for typical PVD-derived films. An in-plane @-scan (Figure 9C) shofi,s ~ h c ~-cia[i\c'

orientation of the cell axes within the c-plane. Reflections appear every 90" as cspccted for an cpiraxial.

' LaAIO, subs!rate (1 10) - Z

* I I

I . . . . I . . . . I . k 0 10 20 30 U) 50 60 70 X O

2 0 (degrees)

telsagonal T B C C O film.

T r a n s p o r ~ measurcmenu provide further indications o f f'ilm properties. T h e I.c51hli~.ir! I 5 .

tempcl.alurc behavior of an MOCVD-derived TI-227-3 film (Figure ] ( ) A ) i n d i c s l c T, = i 5 K. ';L.i.c.l-d]

jillns were pattcsned using standard phorolithogl-aphy and EDTA etching t o dcl'inc an h o pn-I L V ~ ~ ~ ~

microbridge structure. Critical current density results from d c and pulsed transport 1nca5~11.c1ncnt, L I P P ~ ~ I

in Figure 1OB. AL 77 K, the highest J, = 1 . 5 ~ 1 0 5 ,4/crnZ, as defined by a 1 0 pV/cm ot'f'set c~-i~erlcln. BcLII>

!model c a l c u l a ~ i o n s using SQUID susceptometel- data yield J, = 4x106 N c m 2 ar 5 K ( O TJ ~ I I J 6.x I 05

~ c m 2 at 77 K (0 TI. T h e 77 K J, values obtained 1'1-om the transport and lnag~lct ic 1nca.sLlsclnc~~r5 321-cc

reasonably well. suggesting h a t intragrain coupling does not adversely affect rhc tran.\ptrl.l c s p a c i 1 ~ 01

these films. These transport properties are comparable to those of the highest quality P V D - d ~ r i v ~ d ~ ' i l l n .

Figure 1OC shows surface resistance measurements on a i'ilm containing some admixcd TI-22 I ? p h a 5 ~ .

performed in a parallel plate resonator against YBCO. This measurement yields R, = 0.35 m R at 5 K. 1 0

GHz, approaching the lowest values reported for PVD-derived films. MOCVD-derived TI-22 12 rhin

I'ilms are also of high quality with Tc = 105K, and Rs = 0.4 m!2 (SK, 10GHz).

3.5 Growth of dielectric oxide thin films

Many advanced HTS devices require low dielectric loss, lattice-matched thin I'ilms for bui'Ccr Ia~,cl.s 01. I 'OI-

insulating layers in superconductor/insulatodsuperconduc~or (SIS) structures. Ef f ic ien~ k1OCVD grci\\,!Ii

processes for these dielectric materials must be developed along with those ior HTS rna~el-ial.5. I\IOCVU

has heen implemented in the g rowth of oxide sys tems such a s MgO[2X]. YSZ139J. and CcO2[3Oj.

-.---.- , . , . ;". . . * . . . . .2h. . . .2

Temperature (K)

15 0 10 20 30 40 50 60 70 80 30 ID0

Temperature fK)

C5-404 JOURNAL DE PHYSIQUE IV

A. however materials with slnallcr HTS l ~ t t i c c tn is lna~ch~,

-5 -3 - 1 1 3 5 Al(acac)g, and [Ta(OEt)s]-, precursors with a suhsrratc (degrees)

C. temperature of 850°C and NzO as [he oxidizer.l32] 1103) Reflections Films are epitaxial by XRD analysis. ~und high rcsolutioli

LaA103 (1 10) ++

-

i

( < I % ) and greater chemical ~nc r tness arc 01. grcul

- Figure 11A. 0120 XRD scan showing MOCVD

LaA103 by MOCVD using Sr(hl'a)?-tet, Pr(dp~n)? . u n d

derived (001) orielired SrPrGaOl fi i~n grown on G a ( d p r n ) ? precursors with N7O as ~ h c c)xidi;lcr 31

I

. ... . - LaA103. B . @-rocking curve showing FWHM of 0.46" for the (002) retlectio~~. C. @-scan showing in-plane 810"C.[34] The X R D data shown i n Fig111.c I It\

interest.

Recent efforts in this 1ahol.atol.y achicvccl ~ h c i l l -

situ epitaxial growth 01' YAlOz l'ilms on LaAI03

subst ra tes using Y ( d p m ) and A l ( a c a c ) ? a\

precursors.[3 I ] A s ~ ~ b s t r a t c tc~npcraturc 01' XOO'C alitl

N 2 0 oxidizer gas werc ilscd Epi~axy is sccn in thc l'oi11.- 10 20 30 40 50 60 70 80 fold symmetry of the $-scan, and sharp intcl-l'accs a1.c

29 (degrees) B. apparent i n cross-sectional high resolution TEM.

TEM shows no domain houndurics. Also 0 1 ' ~ I . C J I

interest is tetragonal LaSrGaO4 which. along with LI

close YBCO lattice and ~hcl-ma1 expansion cocl'l'icicl~i

m a ~ c h , has no phase transitions ll-0111 low ~~~~~~~~~~~e to

1520"C, thus eliminating twinning.[33] Recently in this

laboratory, thin films of the closely related colnpound.

- .

epitaxy with the 4-fold symmetry of tetragonal indicate phase pure c-axis o r i c n ~ c J SI.prG;1O4 SrPffia04.

LaA103. The sharp rocking curve and 3-l'old sylnlnctry

m 0 *

X 9

- 2 -

d 5b ido 150 2do 250 360 350 rp (degrees) SrPrGaOq, have been grown for the first time (111

of the in-plane scan demonstrate epitaxy (Figures 1 lB-C) .

Orthorhombic PI-Ga03 films have also been grown on LaA103 hy MOCVD using Ps(dprn)? 2nd

L . . , ; ~ . , I 0 -

- CU 0

0

Ga(dpm)? at temperatures of 750-850°C with N20 as the oxidizer. Films arc epitaxial h y XRD, hur thi.

w 0 0 0

subtle (1 1 0 ) and (001) growth domains are again seen in TEM. Howcvc~,. hoth ciolnains have ncul.I!.

However, domains of (1 10) and (001) I'ilm gn)wth arc

also seen by TEM (the rel'lections overlap in XRD

1

identical lattice spacings which arc well-matched to YBCO (0 .1%~) . Onto this hul'icrcd . suhs~l~~~c. .

experiments). The source 01' the ( 1 10) and (001)

domains is the close sirnilal.ity ~n orthol-homhic latticc

parameters. To c i rcumven~ this. the growth oI' cuhic

diclectrics has been invcstiga~cd. TIILIS, Sr2AITaOh

I'ilrns have been grown ill-.\ i r u using Sr(h!'a)2*11't.

.4

X - 4 .- V1 C - U .d

C - -

(002) Full-Width Half Maximum

0.46 @

epitaxial Y B ~ ~ C U ~ O ~ . ~ films have grown by both pulsed laser deposition (PLD) and pulsed

or_rano~netallic beam epitaxy (POMBE). Figures 12A-C shows XRD tla~n dcmon.\[sa[i~ig cl>itasial

I 1 t

I

growth c ~ f ' YBCO b y PLD on thc MOCVD-dcrived epitaxial P I . G ; I O ~ cliclcc~ric laycl.. .Ph;

bupe rcond i~c~ i~ lg p~.opcrtics arc cxccllcnt with T, = 9 1 K and J,[7710 = Ox lo(' ~ / c m ' . l 351 ' f l~i\

A. demonstrates that M O C V D can hc cl.l'cct~vcly ~ ~ a c d In - - - - - - - - - et .-, - the growth of importan1 dicIeciric/s~thst~.atc illma for

HTS applications.

4. CONCLUSIONS

There has been significant progress in lhc clc\.clopment

of M O C V D for H T S applications. T h ~ . o u g h carci'i11

ligand design, volatile. thermally s ~ a h l c Cu. Sr. 2nd Bu 5 15 25 35 45 55

20 (degrees) precursors a re now availahle. I n particular. dcsignccl

B. polyether adducts of fluorinated Ba c o ~ n p l c x c s a1.c 20 = 38.60

(005) Full-Width Hall Maximum liquids under film growth conditions and thus c x h i h i ~ -

0.49 @ + - stable vapor pressure characteristics. For example. the - liquid source, Ba(hfa )ymep, has been implc~nented in * - - .- the successful growth of T B C C O s i~pcrconduc io~ . [hin Y:

U CI films. In the development of fl~1orinc-11-cc B 3 S O L I ~ C C S . r -

J a novel complex has also heen developed in which thc

1 p o l y e t h e r l a r ia t i s c o n n e c l e d to -kctoiminatc

rocking cur"; showing FWHM of 0.49' for (005) T B C C O peIlets a t 820LX95" C . Film transport reflectio~l of Y BCO iudicaring high qualiry. C. @-scau

ir,-planr epiraxy with Lhe subsualc for bolh properlies a re comparahlc to hose ut' the hcst PVD-

-5 -3 -1 1 3 5 frameworks. In undcrstanding the volat i l iza~ion 0 1 ' (degrees)

c. I H T S M O C V D precursors, a novcl thermogravirnc~~.ic

YBCO &d P I - G ~ o ~ . ~ derived films with T, values a s high as 115K and

rp rrnn of <103> plnr~cs of YBCO and <zoo> phnes oc YrGnO,

surface resistances a s low as 0.35mQ at 5 K ( IOGHz).

analysis method has hccn devclopcd. Inl'or~mation

about both vapor pressure and \ ,apor d i f ' i ~ ~ s i o ~ i arc

obtained, and diffusion el'fects are sho\vn LO hc

s ign i f ican t u n d e r typical M O C V D Iiil~n g r o w ~ l i

conditions. B a C a C u O films arc ~ .cad i ly gl'own hy

MOCVD. T h e process is mass transl'cr-li~nitccl at low

temperatures (near 350°C) . and intcrcsting ligand

MOCVD routes to diclcctric oxide films have also been realized with epitaxial YAIO?. S I . ~ A I T ~ O ~ .

5b 160 130 260 230 360 350 41 (degrees) exchange processes are detected. High quality TI-3-223

and 2212 films are formed from BaCaCuO(F) films Figure e ' 2 e XRD scan showillg a annealed i n the presence of T I 2 0 diffusing I'rom hulk YBCO/PI .G~O~/L~A~O~ ~nultilaver structure. B . o-

SrPrGa04, and PrGaO3 growth demonstrated on LaA103. A multilayer structure has been t'uhl-icateci with

high quality epitaxial Y B C O grown upon MOCVD-derived P r G a 0 3 . In summary M O C V D has hecn

~ucccss fu l ly used to g row high quality superconduct ing thin films and dielectric layers which arc

applicahlc to thc t'abl-icalion of HTS-based deviccs

JOURNAL DE PHYSIQUE IV

Acknowledgments

This research was supporled by the National Science Foundation through thc Science a ~ i t i Tcchnologv

Center I'or Superconduc~ivi~y (Granl No. DMR 9120000). by the Northwestern Materials Rcscasch Ccnlcl.

(Grant No. Dh4R 917-052 I ) , and by ARPA (Contract 91-C-0112).

References - -

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